Among automatic focus detection/adjustment methods for image capturing apparatuses, common methods that use a luminous flux passing through a taking lens include a contrast detection method (also called a blur-based method) and phase difference detection method (also called a shift detection method).
The contrast detection method is often used in video movie cameras for movie shooting and electronic still cameras and uses an image sensor as a focus detection sensor. The method pays attention to an output signal of the image sensor, especially to information (contrast information) contained in a high-frequency component and regards the position of the taking lens which maximizes an evaluation value of the output signal as an in-focus point. However, the method is considered to be unsuitable for high-speed focus adjusting operation because the method, which is also known as a hill-climbing method, is required to find an evaluation value by finely moving the taking lens and continue moving the taking lens until the evaluation value turns out to be a maximum value.
The other method, that is, the phase difference detection method, is frequently used for single-lens reflex cameras which utilize silver-halide film and is a technique which has made the greatest contribution to the commercialization of Auto Focus (AF) single-lens reflex cameras. With the phase difference detection method, first a luminous flux passing through an exit pupil of the taking lens is split into two streams, which are then received by a pair of focus detection sensors. Then, by detecting amounts of deviation between signals outputted according to amounts of light received by the focus detection sensors, that is, amounts of relative displacement in a splitting direction of the luminous flux, the phase difference detection method directly determines an amount of deviation of the taking lens in a focusing direction. Thus, once the focus detection sensors accumulate data, an amount and direction of defocus are obtained, enabling high-speed focus adjusting operation. However, in order to split the luminous flux passing through the exit pupil of the taking lens into two streams and obtain signals corresponding to the resulting two luminous fluxes, it is common practice to install light splitting unit such as a quick return mirror or half mirror on an image sensing optical path and install focus detection optics and an AF sensor behind the light splitting means. This has the disadvantage of increasing the size and cost of the apparatus.
To overcome the above disadvantage, a technique has been disclosed which gives a phase difference detection function to an image sensor, eliminating the need for a dedicated AF sensor, and implements a high-speed phase difference AF.
For example, Japanese Patent Laid-Open No. 2000-292686 discloses a technique for giving a pupil division function to an image sensor by splitting light-receiving units of some pixels of the image sensor into two parts. The technique carries out phase difference focus detection by arranging these pixels as focus detection pixels at predetermined intervals in image sensing pixels. Since image sensing pixels are missing in locations where the focus detection pixels are arranged, the technique creates image information by interpolation using information obtained from surrounding image sensing pixels.
On the other hand, even with single-lens reflex cameras, it has become common practice to use solid-state image sensors such as CCD and CMOS sensors as image sensing media instead of silver-halide film. Consequently, products provided with an electronic viewfinder mode and/or movie recording mode as well as an optical finder have appeared on the market. The electronic viewfinder mode captures moving images by retracting quick return mirror from the image sensing optical path, displays the captured moving images on a display device (currently, a liquid crystal display device is used generally) installed in the body, and thereby allows the user to view a subject. Also, such a camera can record not only still images, but also moving images on a recording medium in movie recording mode. During movie capture, which attaches importance to frame rate to enable a smooth screen display, part of pixels used for still images are not read out to improve the frame rate.
On the other hand, to improve image quality during movie display and improve sensitivity at low luminance, Japanese Patent Laid-Open No. 2003-189183 proposes an image sensor capable of producing output by switching between pixel skipping mode and pixel binning mode. That is, in order to improve the image quality of moving images, it is proposed to read in the pixel binning mode for moire reduction when moire is likely to occur because of a high spatial frequency of the subject while using the pixel skipping mode when smear is likely to occur because of high luminance of the subject.
However, the known techniques described above have the following disadvantages.
Normally, a resolution higher than that of still images is not required of the electronic viewfinder and movie modes, for which a smooth screen display is important. Therefore, in the electronic viewfinder and movie modes, it is common practice to improve frame rate by generating images by reading the solid-state image sensor by pixel skipping or pixel binning. In this case, with an arrangement of focus detection pixels described in Japanese Patent Laid-Open No. 2000-292686, there may be cases in which focus detection pixels are not read when the solid-state image sensor is read by pixel skipping, resulting in inability to detect focus using the phase difference method.
It is conceivable to arrange the focus detection pixels in rows (or columns) which are not skipped and thereby ensure that the focus detection pixels will be read even with pixel skipping. However, a technique which switches between the pixel skipping mode and pixel binning mode according to a scene during reading of moving images, such as the technique proposed in Japanese Patent Laid-Open No. 2003-189183, presents the following problem. That is, the focus detection pixels, which can be read properly by pixel skipping, cannot be read properly by pixel binning, in which normal pixels are added to the focus detection pixels.
Furthermore, even in the pixel binning mode, it is conceivable to read only focus detection pixels in the case of pixel groups which contain a combination of normal pixels and focus detection pixels. However, this has a problem in that a wiring layout of control wires will become complicated.